All bunched in close together, orbiting a tiny star, a newly discovered cluster of rocky planets has only just begun to yield its secrets. Richard A. Lovett reports.

Just when we were getting used to the idea of exoplanets – some 3,500 have been discovered since we started looking in 1988 – astronomers have blown us away with an entire new solar system. The star known as TRAPPIST-1 boasts seven rocky earth-sized exoplanets, with at least three close enough to contain liquid water.

Michaël Gillon, an astronomer at the University of Liege, Belgium and colleagues announced the discovery in Nature on 23 February. “It’s beyond anything I could have dreamed of,” says Nikole Lewis, a James Webb Telescope scientist.

TRAPPIST-1 is 39 light years away from Earth. It’s a tiny red dwarf (the most common type of star), only the size of Jupiter and with a mass about 8% of our Sun. With their dim light, red dwarves make planet hunting easier because they significantly reduce in radiance when planets pass in front of them.

In 2010, Gillon’s team first trained a Chilean-based robotic telescope called TRAPPIST ( the Transiting Planets and Planetesimals Small Telescope) on the star. Last May the team announced the discovery of three planets; that number has now been updated to seven. “I was amazed,” Gillon says. “Their very existence was a kind of a shock.”

This chart shows, on the top row, artist conceptions of the seven planets of TRAPPIST-1 with their orbital periods, distances from their star, radii and masses as compared to those of Earth. The bottom row shows data about Mercury, Venus, Earth and Mars.

NASA/JPL-Caltech

Although dim, the star is still able to keep its family warm because they huddle close to it. The inner six planets’ orbits range from 1.51 days to 12.35 days – only a few million miles apart. The outermost orbit, yet to be finalised, is about 20 days.

The planets are also tidally locked, meaning one side permanently faces the star like our Moon – so one side is very hot and the other very cold. Life might have a chance in the narrow band in between.

Although TRAPPIST-1 does not radiate a lot of visible light, its infra-red radiation is 1,000 times brighter – ideal for detection by the Spitzer Space Telescope, which allowed astronomers to measure how the individual planets distorted each other’s orbits. From that, each planet’s mass was calculated and then compared to its size (determined by how much it dimmed the star as it passed). This enabled planets’ density to be computed.

The next step will be to analyse the atmospheres of the planets by looking for spectral fingerprints of gases.

Already, says Lewis, scientists using the Hubble Space Telescope have determined that two of the innermost planets in the system do not have atmospheres dominated by hydrogen or helium. “That’s great, because that’s one more step along the path in having habitable worlds.”